/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/MeshTangents.h
If tangents are available in the overlays, returns them. Otherwise computes orthogonal basis to Normal argument.
void GetTangentVectors ( int32 TriangleID, int32 TriVertexIndex, const FVector3f & Normal, FVector3f & TangentOut, FVector3f & BitangentOut ) const